Type: System..::..DateTimeThe time at which the added stream expires
and is removed from the cache. If absolute expiration is not desired, specify
Cache.NoAbsoluteExpiration

slidingExpiration

Type: System..::..TimeSpanThe interval between the time the added stream was last
accessed and when it expires. If this value is the equivalent of
20 minutes, the object expires and is removed from the cache 20 minutes
after it is last accessed. If sliding expiration is not desired, specify
Cache.NoSlidingExpiration